Triangle $AXY$ is similar to triangle $ZBC$. If $AX = 6$ cm, $ZB = 18$ cm and $ZC = 63$ cm, what is the length of segment $AY$, in centimeters?
Because they are similar, $\frac{AX}{ZB} = \frac{AY}{ZC}$, therefore, $\frac{1}{3} = \frac{AY}{63} \rightarrow AY = \boxed{21}$